1. Field of the Invention
The present invention relates to removal of metal hard mask materials for microelectronic devices. More particularly, the present invention relates to a chemical solution for removing metal hard mask selective to device wiring and dielectric materials.
2. Description of the Related Art
Interconnect circuitry in semiconductor circuits consists of conductive metallic circuitry surrounded by insulating dielectric material. Silicate glass vapor deposited from tetraethylorthosilicate (TEOS) was widely used as the dielectric material, while alloys of aluminum were used for metallic interconnects.
Demand for higher processing speeds has led to smaller sizing of circuit elements, along with the replacement of TEOS and aluminum alloys by higher performance materials. Aluminum alloys have been replaced by copper or copper alloys due to the higher conductivity of copper. TEOS and fluorinated silicate glass (FSG) have been replaced by the so called low-k dielectrics, including low-polarity materials such as organic polymers, hybrid organic, inorganic materials, organosilicate glass (OSG), and carbon-doped oxide (CDO) glass. The incorporation of porosity, i.e. air-filled pores, in these materials further lowers the dielectric constant of the material.
During dual-damascene processing of integrated circuits, photolithography is used to image a pattern on a device wafer. Photolithography techniques comprise the steps of coating, exposure and development. A wafer is coated with a positive or negative photoresist substance and subsequently covered with a mask that defines patterns to be retained or removed in subsequent processes. Following the proper positioning of the mask, the mask has directed there through a beam of monochromatic radiation, such as ultraviolet (UV) light or deep UV (DUV) light (˜250 nm or 193 nm), to make the exposed photoresist material more or less soluble in a selected rinsing solution. The soluble photoresist material is then removed, or “developed,” thereby leaving behind a pattern identical to the mask.
Thereafter, gas-phase plasma etching is used to transfer the patterns of the developed photoresist coating to the underlying layers, which may include hard mask, inter-level dielectric (ILD), and/or etch stop layers. Post-plasma etch residues are typically deposited on back-end-of-the-line (BEOL) structures and if not removed, may interfere with subsequent silicidation, proper metallization or contact formation. Post-plasma etch residues typically include chemical elements present on the substrate and in the plasma gases. For example, if a TiN hard mask is employed, e.g. as a metal hard mask over a dielectric hard mask or as a layer over ILD, the post-plasma etch residues include titanium-containing species, which are difficult to remove using conventional wet cleaning chemistries.
In addition to the need to remove post-plasma residues, it is often desirable to remove or partially etch back the metal hard mask such as a titanium-containing hard mask and/or titanium-containing post plasma etch residue, additional materials that are deposited during the post-plasma etch process such as polymeric residues on the sidewalls of the patterned device and copper-containing residues in the open via structures of the device are also preferably removed. No single wet cleaning composition has successfully removed all of residue and/or hard mask material while simultaneously being compatible with the ILD, other low-k dielectric materials, and metal interconnect materials. Compositions in the art claim to act in such a manner but are extremely less effective than the claims indicate.
The integration of new materials, such as low-k dielectrics, into microelectronic devices places new demands on cleaning performance. At the same time, shrinking device dimensions reduces the tolerance for changes in critical dimensions and damage to device elements. Etching conditions can be modified in order to meet the demands of the new materials. Likewise, post-plasma etch cleaning compositions must be modified. Importantly, the cleaner should not damage the underlying dielectric material or corrode metallic interconnect materials, e.g. sensitive ILD materials such as carbon-doped oxides and metal structures such as copper, tungsten, cobalt, aluminum, ruthenium and silicides thereof, on the device.
Typical trench first metal hard mask integration removes the metal hard mask during the chemical mechanical polish process that removes excess device metallurgy. As integration tolerances tighten, the limited ability to correctly fill the defined metal receiving structures has been clearly demonstrated.
Additional complications arise when a self-aligned via (SAV) process that requires enhanced metal hard mask stability is used to provide additional lithographic process window. While it may be beneficial for metal fill to add trapezoidal cross-sectional character to an integration structure, line to line integration space can suffer if an excessive trapezoidal cross-sectional design is used to enhance metal fill of very high aspect structures. A metal hard mask can be designed such that the lithographic transfer into the metal hard mask will define the desired future trench structure and yet be resistant to undesired damage during reactive ion etch transfer operations into the ILD structures such that a metal fill definition structure may be constructed without significant trapezoidal character. An unfortunate byproduct of this aforementioned process is an increase in aspect ratio, which may further impede proper metallization.
What is needed to advance new technologies is a method to improve the aspect ratio for metal deposition while still maintaining the desired line to line integration spaces. U.S. Pat. No. 7,922,824 suggests the use of quaternary ammonium salts and quaternary ammonium alkali as part of a chemical composition for removing metal hard masks and post-plasma etch residues. However, it teaches away from the use of quaternary ammonium salts and quaternary ammonium alkali without the addition of an acid modifying agent, such as citric acid, and by this teaching as well as the direct omission of quaternary ammonium salts in the list of oxidizing agent stabilizers indicates that quaternary ammonium salts and quaternary ammonium alkali cannot be used alone.